We teach people to normalize their breathing so as to improve their sleep and their health.
You may wish to see if you recognize any of the symptoms commonly associated with dysfunctional breathing https://learntosleepwell.com/checklist-of-symptoms/
If you can breathe through your nose during the day you will increase your chances of doing so through the night. There are numerous health benefits in nose breathing*. Unfortunately, these benefits have been greatly neglected by physicians according to Dr Pat Barelli, Otolaryngologist 
You may meditate for 15 – 20 minutes**. Or you may read or listen to music as part of your winding down in preparing for sleep.
Sleeping on your back promotes deeper mouth breathing whereas sleeping on your side gives you greater control over your breathing
This position facilitates relaxed and open airways while supporting a closed mouth during sleep
Our body’s core temperature needs to drop by 1°C to initiate the evening surge of melatonin prior to facilitating sleep
Digesting a large quantity of food in the evening will increase your heart rate and, in turn, your breathing. Stimulants, particularly caffeine, can take up to 8 hours to wear off
Although alcohol acts as a sedative in relaxing you it deprives you of REM sleep and keeps you in the lighter stages of sleep. Heavy alcohol consumption may impair your breathing at night. You will also likely wake up in the middle of the night when the effects of alcohol have worn off.
Sleep patterns may be interfered with by commonly prescribed heart, blood pressure or asthma medications or over the counter remedies for colds, coughs and allergies. Check with your Doctor as to whether any side effects exist insofar as sleep is concerned and if the medication can be changed or taken in the morning.
Exercise benefits your cardiovascular system, respiratory system and nervous system but exercise should not occur less than 2 hours before sleep. Morning sunlight exposure is preferable in regulating our circadian rhythm***
*There may be valid reasons why nose breathing is either difficult or not possible, for example
All of the above (except 1 and 3) can be assisted significantly by mindfully retraining how you breathe. A tongue-tie release will greatly assist breathing difficulties associated with 1 and 3
** During mindfulness meditation it is helpful to focus on either your breath or on sounds and use them as anchors to the present. When you are focused on the present – on what is happening right now – it is not possible for worries and anxieties to come charging into your head. If thoughts do intrude simply acknowledge the thought and return to your anchors
***Not all of us have the same circadian rhythm. About 40% are morning types and about 30% are evening types. The remaining 30% lie somewhere in between morning and evening types
 Dr Barelli, Pat A 1987, Behavioural and Psychological Approaches to Breathing Disorders, Plenum Press, New York 1994
 Matthew Walker, 2018, Why We Sleep, The New Science of Sleep and Dreams, Penguin at p. 275
 Matthew Walker, 2018, Why We Sleep, The New Science of Sleep and Dreams, Penguin at p. 341
 Gill Hasson, 2013, Mindfulness, Capstone at p. 27
 Matthew Walker, 2018, Why We Sleep, The New Science of Sleep and Dreams, Penguin at p. 20